Copenhagen Researchers Develop Tunable Quantum System for Enhanced Sensing

Researchers at the Niels Bohr Institute, University of Copenhagen, have unveiled a groundbreaking tunable quantum system that promises to enhance sensing capabilities across various technological fields, including biomedical diagnostics. This innovative system, detailed in a study published in *Nature* on July 3, 2025, introduces a novel approach to surpassing the standard quantum limit, which has traditionally constrained the sensitivity of optical sensing technologies.

The potential applications for this tunable system span a wide range, from detecting gravitational waves in the cosmos to sensing minute physiological changes in human bodies. Optical sensing technologies have become integral to everyday life, advancing significantly due to quantum optics, which has pushed the boundaries of device sensitivity close to the standard quantum limit. This limit arises from unavoidable noise when measuring at the smallest scales, necessitating the adoption of sophisticated quantum techniques to mitigate or cancel out this noise.

Key concepts enabling this advancement include squeezed light, back-action evasion, and entanglement. According to Professor Eugene Polzik, a leading researcher at the Niels Bohr Institute, "The sensor and the spin system interact with two entangled beams of light. After the interaction, the two beams are detected and the detected signals are combined, resulting in broadband signal detection beyond the standard quantum limit of sensitivity."

Historically, entanglement has been observed in microscopic systems, like individual atoms and photons. However, the system developed by the Niels Bohr Institute employs large-scale entanglement, marking a first in the integration of a multi-photon light state with a large atomic spin ensemble. This unique combination facilitates frequency-dependent squeezing, allowing dynamic reduction of quantum noise over a broad frequency band, which is vital for applications such as gravitational wave detection and various other sensing technologies.

The researchers explain that conventional methods for achieving frequency-dependent squeezing and quantum noise reduction typically require extensive, complex optical setups. For instance, gravitational wave detectors like LIGO in the United States and VIRGO in Italy utilize 300-meter-long optical resonators to achieve similar outcomes. The new method, however, demonstrates the possibility of achieving comparable performance with a compact tabletop device, paving the way for more practical applications.

The implications of this hybrid quantum network for sensing applications are vast. Potential uses include advanced sensors for detecting minute alterations in magnetic fields, time, or acceleration. In the biomedical realm, such sensors could significantly enhance the resolution of magnetic resonance imaging (MRI), enable early detection of neurological disorders, and improve the sensitivity of biosensors utilized in diagnostics and monitoring.

Valeriy Novikov, the lead author of the study, emphasizes that their research could enhance the sensitivity of gravitational wave detectors, allowing for the detection of faint ripples in spacetime—signals indicative of cataclysmic cosmic events such as black hole mergers and neutron star collisions. A deeper understanding of gravitational waves will also contribute significantly to theories regarding the formation processes of the universe.

Beyond its sensing capabilities, the architecture of this new system also opens doors to advancements in quantum communication and computation. It could potentially be adapted for use in quantum repeaters, enhancing secure long-distance communication signals, and in quantum memories within quantum networks. Thus, this tunable quantum system holds promise across multiple domains of quantum technology, indicating a significant leap forward in both theoretical and practical applications.

In summary, the development of this tunable quantum system at the Niels Bohr Institute exemplifies the intersection of advanced physics and practical technology, offering exciting prospects for future research and application across diverse fields. The full study can be accessed in *Nature* (DOI: 10.1038/s41586-025-09224-3).